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Why is Pluto no longer a Planet?

Pluto , discovered in 1930 by an American astronomer Clyde Tombaugh was known to us as the farthest and smallest planet which was placed ninth in the solar system. This was exactly how every science text book read, until 2006 when some changes led to Pluto being removed from the list of planets.
So, what happened in 2006? Why the no. of planets came down from 9 to 8. Lets find out what changed and why it happened.

What changed in 2006?

In 2006 , the definition of Planet was changed. For any celestial body to be considered as a planet , it must follow the three rules.

  • It must orbit around the sun.
  •  It needs to have enough gravity to pull itself into spherical shape.                                                    
  • It needs to have “cleared the neighborhood” of its orbit.

So, Pluto did meet the first two criteria as its round in shape and also orbits around the sun. But, it failed to follow the third rule, as it didn’t become dominant body to clear its surroundings. This was because its mass wasn’t large enough to clear all objects in its surroundings.

What led to these changes ?

Change in the definition of planet wasn’t an overnight decision. There were series of events that led to these change. The astronomers all around the world were doing studies to find out celestial objects past Neptune which were similar to Pluto in mass and properties. This space past Neptune was called as Kuiper belt. It extends from about 30 to 55 astronomical units (compared to Earth which is one astronomical unit, or AU, from the sun). After years of research the astronomers were able to locate number of objects which were almost similar to pluto. Some objects were Quaoar (announced in 2002), Sedna (2003) and Eris (2005). In fact, the astronomers went on to say that there were around 70,000 icy objects which had similar composition as Pluto.

So, due to these new findings the International Astronomical Union had to take a call if they were to increase the total no. of planets or to change the criteria for celestial body to be referred to as Planet. In August , 2006 the members of IAU voted in favor of changing the definition of planet and came up with the three rule criteria.
Along with this , they created the new classification of “Dwarf Planet” , the planets which fulfilled only two criteria. So now, Pluto along with Makemake, Haumea and Eris falls in the category of dwarf planet.

Kuiper_whatwhenwhowhy
Dwarf Planets in Kuiper Belt

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